Pollutants in combustion exhausts from low-sulfur fuels are generally comprised of carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (“NOx”, which generally comprises a mixture of NO and NO2). Currently, for spark-ignited, fuel-injected passenger car engines, a three-way catalyst (TWC) is employed that greatly reduces the levels of all three of these pollutants. The TWC is only effective within a narrow range of oxygen (O2) concentrations in the exhaust, losing its effectiveness for NOx removal at higher O2 contents. Therefore the TWC cannot generally be employed for NOx remediation of exhausts from diesel and lean-burn gasoline engines, which tend to be O2-rich.
Engines that produce O2-rich exhausts require on-board NOx remediation with techniques such as selective catalytic reduction (SCR) with reagent (HC and/or urea) injection. The amount of reagent injection during SCR is critical, as enough must be supplied to completely decompose the NOx, but the addition of excess must be avoided. Therefore, it is essential to develop sensors that can rapidly and accurately assess the NOx levels in these exhausts and enable improved emissions control and on-board diagnostics. NOx sensors suitable for these applications should be compact, robust, low-cost, capable of operating at temperatures near 600-700° C., and able to measure NOx in the concentration range ˜0 ppmV≦[NOx]≦1500 ppmV at O2 levels varying between 5 and 20%.